1. Field of the Invention
This invention relates to a tachometer for measuring the speed of a periodically varying position signal that has a slope representative of speed and more particularly to a tachometer for producing a signal representative of the speed at which a magnetic head moves radially of a magnetic disc.
2. Description of the Prior Art
In disc data storage systems, data is stored on concentric tracks on a magnetic disc surface. In order accurately and quickly to access the data stored on a prescribed track, it is imperative that the speed of radial movement of the magnetic heads be measured continuously and accurately. This is recognized in the known prior art and various tachometers for producing velocity information are disclosed.
Among the known tachometer systems are those utilizing an independent transducer such as a magnetic transducer, those using an independent position transducer such as an optical system (e.g., U.S. Pat. No. 3,568,059 [cl. 324/177] to Sordello and U.S. Pat. No. 3,811,091 [cl. 328/1] to Ha et al.), those deriving a position signal from the disc pack and differentiating that signal with respect to time (e.g., U.S. Pat. No. 3,534,344 [cl. 340/174.1] to Santana), those integrating the motor current which is proportional to acceleration (e.g., Sordello, supra), and those employing two or more of the above enumerated expedients in combination. These prior art tachometers employ analog circuits to perform the velocity calculation. Not only are sophisticated analog circuits complex, but they are also limited in frequency response. The limitation of frequency response increases in importance as advancements in disc technology make possible greater and greater densities of data storage.